What are wood pellets?
Wood pellets are made from dry sawdust compressed under high pressure and extruded through a die. They may include a low level of added binder, such as starch, but many use nothing other than steam.
They come in a range of sizes: for domestic and relatively small scale systems 6 or 8 mm is typical, while for larger systems 10 or 12 mm are common.
Wood pellets should be dry, clean, mechanically robust and have an ash content defined by the appropriate standard (see below) to which they have been made, which may also define other contaminants such as chlorine content.
They should also flow freely and can be delivered via a pneumatic system.
What are briquettes?
Briquettes are similar to wood pellets, but physically larger. Sizes vary but briquettes can vary in diameter from around 50 mm to 100 mm +. Briquettes are usually between 60 mm and 150 mm in length. They can offer a cleaner, more consistent alternative to firewood logs, offering higher energy density and steady combustion.
The combination of low moisture content (8-10%) with a compressed, dense pellet (typically >1,000 kg/m3, i.e. greater than the original wood) and the ability to flow and pack closely, gives a fuel with a high bulk energy density. This then requires less storage space than lower density fuels such as wood chips.
Using as a biofuel
Wood pellets are also a pleasant, clean, consistent fuel, and modern pellet boilers are highly sophisticated, featuring all the facilities available on modern gas boilers.
When combined with a reliable supply, infrastructure wood pellets have proved to be a very popular renewable alternative to fossil fuel based domestic, and small to medium scale district and site heating in several European countries.
Using as an alternative to coal
Wood pellets also provide an alternative, renewable and low carbon alternative to fuel old coal boilers.
Commercial conversion kits are available, but some systems are being run with few modifications other than to the fuel feed which needs to be higher owing to the lower energy density of wood pellets.
Where coal is in use storage and handling infrastructure for a solid fuel will already be in place. Wood pellets provide a fuel that, in addition to its environmental benefits, is cleaner, more pleasant to handle, is less aggressive to fuel handling components such as augers, and produces considerably less ash.
A coal storage bunker will hold enough pellets for operation for around one half to a third of the time possible with the same volume of coal.
Wood pellets are only as good as the standard to which they are manufactured. Poor quality pellets may display inferior mechanical robustness, and will tend to disintegrate into sawdust more readily than those of better quality. They may also have lower energy density, higher levels of ash or contaminants such as heavy metals, chlorides or sulphur or higher moisture content.
There are a number of established standards in the EU:
- ÖNORM M1735 in Austria
- SS 187120 and SS 187121 in Sweden and DIN 51731
The EU-wide CEN standard EN 335 for solid biofuels is in preparation, which includes wood pellets.
Wood pellets have a higher embedded energy than woodchips, owing to the energy needed to grind the wood to fine sawdust (typically in a hammer mill) and press it through a die at high pressure. The energy (and financial) cost of active drying is not usually justified.
However a very low moisture content (typically <10%) is required of the sawdust, so many pellet production facilities are established where there is an existing supply of very dry material, such as were processing of wood that is already kiln dried takes place. For example:
- Joinery workshops
- Wood recycling sites
- Sawmills processing dried wood.
This also removes the need to transport the very low density sawdust. Where a supply of bone dry sawdust is available, this may be supplemented with sawdust of higher moisture content provided the overall moisture content does not become too high. The precise embedded energy figure for a batch of pellets will depend on the details of production (plus that of the feedstock), transport etc.